A simple way to do this would be using java.text.DecimalFormat:

fun formatter(n: Int) =
    DecimalFormat("#,###")
        .format(n)
        .replace(",", ".")

println(formatter(1000000)) // => 1.000.000
println(formatter(750000))  // => 750.000

However, do note that the dot (.) symbol is commonly used as a Decimal separator (or monetary decimal separator)

So I would advice using , instead of of ..

Try it online!

Update:

If your intention is having a German locale (Thanks @AndrewL) formatting, then configure the DecimalFormatSymbols with the correct locale:

fun formatter(n: Int) =
    DecimalFormat("#,###", DecimalFormatSymbols(Locale.GERMANY)).format(n)

Try it online!

Even-though It's late. Intended for future visitors.

Fetures of the following codes

1. Puts thousand separator in EditText as it's text changes.

2. adds 0. Automatically when pressed period (.) At First.

3. Ignores 0 input at Beginning.

Just copy the following Class named

NumberTextWatcherForThousand which implements TextWatcher

import android.text.Editable;
import android.text.TextWatcher;
import android.widget.EditText;
import java.util.StringTokenizer;

/**
 * Created by skb on 12/14/2015.
 */
public class NumberTextWatcherForThousand implements TextWatcher {

    EditText editText;


    public NumberTextWatcherForThousand(EditText editText) {
        this.editText = editText;


    }

    @Override
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {

    }

    @Override
    public void onTextChanged(CharSequence s, int start, int before, int count) {

    }

    @Override
    public void afterTextChanged(Editable s) {
        try
        {
            editText.removeTextChangedListener(this);
            String value = editText.getText().toString();


            if (value != null && !value.equals(""))
            {

                if(value.startsWith(".")){
                    editText.setText("0.");
                }
                if(value.startsWith("0") && !value.startsWith("0.")){
                    editText.setText("");

                }


                String str = editText.getText().toString().replaceAll(",", "");
                if (!value.equals(""))
                editText.setText(getDecimalFormattedString(str));
                editText.setSelection(editText.getText().toString().length());
            }
            editText.addTextChangedListener(this);
            return;
        }
        catch (Exception ex)
        {
            ex.printStackTrace();
            editText.addTextChangedListener(this);
        }

    }

    public static String getDecimalFormattedString(String value)
    {
        StringTokenizer lst = new StringTokenizer(value, ".");
        String str1 = value;
        String str2 = "";
        if (lst.countTokens() > 1)
        {
            str1 = lst.nextToken();
            str2 = lst.nextToken();
        }
        String str3 = "";
        int i = 0;
        int j = -1 + str1.length();
        if (str1.charAt( -1 + str1.length()) == '.')
        {
            j--;
            str3 = ".";
        }
        for (int k = j;; k--)
        {
            if (k < 0)
            {
                if (str2.length() > 0)
                    str3 = str3 + "." + str2;
                return str3;
            }
            if (i == 3)
            {
                str3 = "," + str3;
                i = 0;
            }
            str3 = str1.charAt(k) + str3;
            i++;
        }

    }

    public static String trimCommaOfString(String string) {
//        String returnString;
        if(string.contains(",")){
            return string.replace(",","");}
        else {
            return string;
        }

    }
}

Use This Class on your EditText as follows

editText.addTextChangedListener(new NumberTextWatcherForThousand(editText));

To get the input as plain Double Text

Use the trimCommaOfString method of the same class like this

NumberTextWatcherForThousand.trimCommaOfString(editText.getText().toString())

Git

You can include locale-specific group separators with the formatting specifier ,. I would first try to write it this way:

binding.resultId.text = String.format("%,f", DigitOnScreen.toString().toDouble())

Then to get the right number of decimals, i.e. corresponding to the user input, you have to count the digits after the decimal separator. Here you explicitly use . as the decimal separator for DigitOnScreen.

val numberAsString = DigitOnScreen.toString()
val nofDecimals = numberAsString.substringAfter('.', "").length
binding.resultId.text = String.format("%,.${nofDecimals}f", numberAsString.toDouble())

Note also that in your case, DigitOnScreen is not the number as seen on the screen, and that on screen (what is in binding.resultId.text) the group and decimal separators will depend on the current app's locale.